Another name for a
hidden terrorist or agent, one who hides among
hostages until the time is right.
Picture this, a plane has just been hijacked by terrorists. Now, there was actually an FBI agent posing as a passenger, who at the right moment jumps up and overpowers the hijacker. See? He's what they call a sleeper. A number of European Airlines had this when there was a threat.
Usually it's a terrorist who is the sleeper. If you saw the movie Executive Decision, there was a bomb on board, controlled by the sleeper. If troops boarded the plane and took out the terrorists, the 'other' terrorist/sleeper, posing as a passenger, could then either get up and suprise-attack the troops, or set off the bomb.
In light of the World Trade center terrorism, the US Government has said they will have U.S. Marshalls on board incognito trained in anti-hijacking techniques may travel on all flights.